Moving AutoBisectApi.working_dir fallback logic to the property code.

De-duplicating the code to increase readability. It is also worth noting
that the only remaining use for this fallback is for the benefit of
recipe simulation test.

R=phajdan.jr@chromium.org,sullivan,prasadv@chromium.org,dtu@chromium.org
BUG=636499

Review-Url: https://codereview.chromium.org/2230283004
3 files changed